Semiconductor manufacturing, such as the fabrication of integrated circuits, typically entails the use of photolithography. A semiconductor substrate on which circuits are being formed, usually a silicon wafer, is coated with a material, such as a photoresist, that changes solubility when exposed to radiation. A lithography tool, such as a mask or reticle, positioned between the radiation source and the semiconductor substrate casts a shadow to control which areas of the substrate are exposed to the radiation. After the exposure, the photoresist is removed from either the exposed or the unexposed areas, leaving a patterned layer of photoresist on the wafer that protects parts of the wafer during a subsequent etching or diffusion process.
The photolithography process allows multiple integrated circuit devices or electromechanical devices, often referred to as “chips,” to be formed on each wafer. The wafer is then cut up into individual dies, each including a single integrated circuit device or electromechanical device. Ultimately, these dies are subjected to additional operations and packaged into individual integrated circuit chips or electromechanical devices.
A similar process is used to manufacture read write components for use in data storage drives. In a typical data storage drive, the data is stored on round, flat disks called platters, usually made of glass or an aluminum alloy. Special electromagnetic read/write devices called heads are used to either record information onto the disk or read information from it. The read/write heads transform electrical signals to magnetic signals, and magnetic signals back to electrical once again. Each bit of data to be stored is recorded onto the hard disk using a special encoding method that translates zeros and ones into patterns of magnetic flux reversals.
The portion of a write head that actually writes data on the disk is referred to as the write element. This element is typically made up of two poles that are separated by a gap. These poles generate a magnetic field when they are excited by a coil magnetically coupled to the poles. When the write element is in proximity to the disk, a magnetic field generated by the poles sets the magnetic orientation in given locations on the disk. In this manner, data is written on the disk.
As the computer industry continues to demand higher capacity and faster performance from hard disks and tape drives, there is an increasing demand for suppliers to increase the amount of data that can be stored on a given storage medium. This amount of data, referred to as areal density, is usually expressed as the number of bits of data per square inch of storage media. One of the major factors that determines the areal density of a hard disk is the track density. This is a measure of how tightly the concentric tracks on the disk can be packed. Track density is largely determined by the width of the tracks, which is in turn largely determined by the width of the write element. A large write element will affect a larger area on the surface of a platter than will a smaller write element. As a result, track width can be decreased (and track density increased) by making the poles of the write head physically narrower, especially at the write tip, thereby concentrating the magnetic field into a smaller area on the platter surface.
A large percentage of the write heads used today are thin-film heads, so named because of the way in which they are manufactured. During the manufacturing process, a substrate wafer is coated with one or more layers of a very thin film of alloy material deposited in specific patterns. Alternating layers of an insulating material are also deposited onto the substrate. Lithographic techniques similar to those used to manufacture semiconductor circuits are used to form the deposited layers into a pole-tip assembly having the desired geometry.
During the manufacturing process, variations in exposure and focus require that the patterns developed by lithographic processes be continually monitored or measured to determine if the dimensions of the patterns are within acceptable ranges. The importance of such monitoring, often referred to as process control, increases considerably as pattern sizes become smaller, especially as minimum feature sizes approach the limits of resolution available by the lithographic process. Typically, for structures such as write heads, the width dimension is one of the smallest feature dimensions, and it is the width dimension that is conventionally monitored to assess performance of the lithographic process.
Monitoring of pattern features and measurement of its dimensions, commonly referred to as metrology, is often performed using a charged particle beam system, such as a focused ion beam system (FIB), in conjunction with a scanning electron microscope (SEM).
During a typical metrology process, a FIB system is used to expose the cross-section of a structure, such as a write head, so that the structure width can be accurately measured. FIB systems are widely used in microscopic-scale manufacturing operations because of their ability to image, etch, mill, deposit, and analyze very small structures with great precision. FIB systems produce a narrow, focused beam of charged particles (hereinafter referred to as ions) that is typically scanned across the surface of a work piece in a raster fashion, similar to a cathode ray tube. In most commercial FIB systems, the ions used are positively charged gallium ions (Ga+) extracted from liquid metal ion sources. The extracted ions are accelerated, collimated, and focused onto a work piece by a series of apertures and electrostatic lenses. The ion beam can be used to remove material from the work piece surface or to deposit material onto the surface. When used to remove material, often referred to as milling, the heavy gallium ions in the focused ion beam physically eject atoms or molecules from the surface by sputtering, that is, by a transfer of momentum from the incoming ions to the atoms at the surface.
Because FIB sputter-milling can cause significant damage to small structures, the structure surface is often coated with a protective layer of tungsten before milling begins. Such a layer can be deposited using a gas that decomposes in the presence of the ion beam and deposits material onto the surface. This process is commonly referred to as FIB-induced chemical vapor deposition (CVD). Typically the precursor gas, such as tungsten hexacarbonyl gas, is directed onto the work piece surface, usually via a fine needle inserted near the position of the ion beam. The gas is broken down into volatile and non-volatile components by the low energy electrons generated when the ion beam strikes the surface. The non-volatile component, in this case the protective tungsten coating, is deposited onto the surface, while the volatile component is pumped away.
Once the cross-section is exposed, a scanning electron microscope can be used to measure the width of the structure. The scanning electron microscope (SEM) allows for the production of an image of greater magnification and higher resolution than can be achieved by the best optical microscopes. An SEM produces a finely focused beam of electrons which is scanned across the surface of a work piece, typically in a raster pattern. The electrons that make up the electron beam are called primary electrons. When the electron beam is directed at the work piece surface, the primary electrons collide with electrons in orbit around the nuclei of the atoms present in the work piece causing the emission of secondary electrons. Some of the primary electrons will also be reflected from the work piece surface. These higher energy electrons (>50 eV) are called backscattered electrons. Both types of electrons can be detected by inserting an appropriate detector near the specimen. The detector produces a variable voltage output; the more secondary or backscattered electrons it detects, the greater will be the voltage generated.
The analog signal produced by the detector is typically converted into a digital brightness value by a device known as an Analog to Digital converter. The voltage of the detector's output signal is measured for each point in the scan (commonly referred to as a pixel) and assigned a number from representing a digital brightness value for that pixel. Although the gray-level resolution (the number of shades of gray used in the image) can be varied, typically 256 gray levels are used, so that each point is assigned a value from 0 (black) to 255 (white) according to the intensity of the voltage. The value for each pixel is stored in a memory array and used to produce a multilevel gray image of the target surface in which the brightness of each point on the image is determined by the number of secondary or backscattered electrons ejected while the primary electron beam was impinging at that point. Areas where a relatively low number of electrons are emitted will appear darker in the image, while areas where a relatively high number of electrons are emitted will appear brighter.
The average number of secondary electrons produced per primary electron is called the secondary-electron coefficient (SEEC), and is typically in the range 0.1 to 10 (varying between different materials). The average number of backscattered electrons reflected per primary electron is called the backscattered-electron emission coefficient (BEEC). The number of electrons emitted at a given pixel depends on many factors, such as the topography of the sample, the curvature of the target surface, the electron emission coefficient of the target material, and even (especially for backscattered electrons) the atomic number of the elements present in the sample. Because different materials may have significantly different electron emission coefficient values, the yield of emitted electrons, whether secondary or backscattered detection is employed, may be used as a contrast mechanism to distinguish between different materials on a surface—especially where the difference between the electron emission coefficients of the two materials is relatively high.
Typically, to measure the width of cross-section of a structure, the SEM is used in conjunction with automatic metrology software. As the electron beam is scanned across the exposed cross-section, whether secondary or backscattered detection is employed, there will typically be a change in electron intensity at the edges of the structure. This change can be due to a change to topography or to a transition between two different materials. An algorithm is used to assign an edge position based upon the contrast at the edges of the structure and to determine the distance between those edges. Thus, the accuracy of the algorithm's edge position determination determines the accuracy of the width measurement.
Unfortunately, there are a number of problems when the above-described processes are used to monitor the width of very small structures such as modern write-head poles. SEM image based metrology relies upon the ability to make measurements between gray level transitions. Gray level variations in an SEM image can either result from changes in topography or from material differences. For structures such as the write heads discussed above (which are covered with a protective overcoat prior to FIB cross-sectioning) the edges that must be detected and measured are formed solely by material differences between the pole structure and its overcoat material. In order to accurately measure the distance between two such material boundaries, it is desirable to reduce or eliminate any variation in topography so that measurements are based entirely upon the difference in material.
When a FIB system is used to mill a cross section of a selected structure of interest which has been coated with a protective layer of a second material, a phenomenon known as “curtaining” often affects the accuracy of any subsequent SEM measurement. FIG. 1A is a SEM micrograph of a cross-section of a typical NiFe write-head with a tungsten overcoat. Write-head 150 is composed of a compound known as Permalloy—an 81/19 alloy of nickel and iron. Overcoat 140 is composed of tungsten deposited to protect the write-head during FIB milling and to provide the necessary gray level difference for edge-finding and width measurement of the pole. As illustrated by FIG. 1A, the edges of the write-head in areas 120 and 130 are not well defined because artifacts from the milling process associated with using tungsten as the overcoat material partially obscure the material boundaries. This is commonly referred to as curtaining.
FIG. 1B is a SEM micrograph of the same cross-section observed from a slight angle with respect to the vertical face of the cross-section. As can be seen in area 132, the Permalloy write-head 150 is actually recessed slightly with respect to the tungsten layer 140. During ion beam milling, sputtered material can redeposit on the workpiece surface. A recess, as shown in FIG. 1B, tends to collect redeposited material during the milling process. The resulting variation in topography results in gray level variations in an SEM image that partially obscures the boundary or edge between the Permalloy write-head and the tungsten overcoat layer. The poorly defined transition between the Permalloy and the tungsten results in the curtaining effect discussed above. Curtaining can make edge recognition difficult and can possibly lead to a less accurate cross-section measurement by automated metrology software.
One approach to the problem of curtaining is the use of a tilted ion beam to mill the cross-section. The workpiece is then rotated 90 degrees, and the ion beam is used to remove some of the redeposited material. This approach, however, suffers from a number of shortcomings. First, because the cross-section is milled at an angle, the actual vertical cross-section measure must be calculated from the angled measurement. The resulting width determination is less accurate than if the vertical cross-section were to be measured directly. Further, the process takes significant additional time because the cross-section must be milled, the sample rotated, and then the redeposited material must be removed before an SEM image can be taken and measurements can be performed. Any increase in time required to complete a critical dimension measurement process is very undesirable for any in-line process control.
Thus, there is still a need for an improved method of controlling topographical variations when milling a cross-section of a structure such as a write head pole. A reduction in topographical variation will produce a more planar cross-section face and accordingly can improve the accuracy of metrology applications such as measuring a width of the cross-section of such a structure.